The invention relates to an audio scrambler which can be used singly, or in combination with, a video scrambler.
Signal scrambling techniques are known in audio as well as in video systems for the purpose of preventing unauthorized hearing or viewing. In contrast to a cryptographic security system, a scrambler system does not necessarily provide total privacy in the communication. It does not insure that the message will not be unscrambled by somebody determined to do it. At least scrambling will make it difficult, or costly, for someone to reconstitute the scrambled message without the knowledge of the code.
The present invention adapts for audio scrambling a technique which has been disclosed in a television system for video scrambling. The technique is found in U.S. Pat. No. 4,070,693 issued Jan. 24, 1978 to Harold B. Shutterly. While the scrambling method in the patent is applied exclusively to a television video signal, it affords several advantages when, as proposed by the present invention, it is adapted for audio scrambling in general, and more particularly, used for transmitting the audio on the video channels of a television system.
It is known in the prior art to concurrently scramble the audio and the video signals of a television system. See for instance U.S. Pat. No. 3,919,462 of A. Hartung et al., No. 3,852,519 of Patrick R. J. Court and No. 3,736,369 of Joseph H. Vogelman et al.
It is known from U.S. Pat. No. 3,872,503 of H. B. Shutterly, to divide a continuous signal into successive segments, or samples and with these to generate time-compressed audio signals in the video format which can be accommodated with true video signals for concurrent use, e.g. for transmission in a television system. Discontinuities and unwanted transients at the junction between segments are eliminated from the constructed signal by adding repeated portions of the segment, or sample, at the ends thereof.
It is known from U.S. Pat. No. 3,789,137 of George F. Newell to time-compress segments of an audio signal to convert them into video format for recording, or transmission. Analog-to-digital conversion and digital to analog conversion at both ends of the conversion scheme are used with two registers for alternately processing in digital form successive segments divided from the original continuous audio signal.
It is known from U.S. Pat. No. 3,731,197 of J. E. Clark to divide a continuous audio signal into successive segments which are sampled for storage and scrambling in accordance with a code before transmission. The same code at the receiving end allows reconstruction of the segments and continuous reproduction of the audio signal.
It is known from U.S. Pat. No. 3,824,467 of R. C. French, or U.S. Pat. No. 3,970,790 of G. Guanella, to divide an audio signal into segments and to rearrange such segments so as to form an unintelligible transmitted signal.
It is known from U.S. Pat. No. 3,925,612 of G. Guanella, et al. to scramble a polarity modulated message pulse series with a shift register which is scrambled cyclically, scrambling being obtained by output feed back into selected locations of the shift register.
It is known from U.S. Pat. No. 3,819,852 of Peter Wolf to store the audio in time-compressed form during the period of a line in the vertical blanking interval subsequently to the video signal.